1. Field of the Invention
The present invention relates generally to composites that may be used in energy storage devices.
2. Description of the Prior Art
Electrochemical capacitors (also denoted as supercapacitors or ultracapacitors) are a class of energy-storage materials that offer significant promise in bridging the performance gap between the high energy density of batteries and the high power density derived from dielectric capacitors. Energy storage in an electrochemical capacitor is accomplished by two principal mechanisms: double-layer capacitance and pseudocapacitance. Double-layer capacitance arises from the separation of charge that occurs at an electrified interface. With this mechanism the capacitance is related to the active electrode surface area, with practical capacitances in liquid electrolytes of 10-40 μF/cm2. Electrochemical capacitors based on double-layer capacitance are typically designed with high-surface-area carbon electrodes, including carbon aerogels, foams, and papers. (Frackowiak et al., Carbon, 39, 937 (2001). All referenced patents and publications are incorporated by reference.) Carbon aerogels are particularly attractive due to their high surface areas, high porosities, and excellent conductivities (>40 S/cm) (Pekala et al., J. Non-Cryst. Solids, 225, 74 (1998)).
Pseudocapacitance broadly describes Faradaic reactions whose discharge profiles mimic those of double-layer capacitors. Because this mechanism involves true electron-transfer reactions and is not strictly limited to the electrode surface, materials exhibiting pseudocapacitance often have higher energy densities relative to double-layer capacitors. The main classes of materials being researched for their pseudocapacitance are transition metal oxides, conductive nitrides, and conducting polymers. At present, some of the best candidates for electrochemical capacitors are based on nanoscale forms of mixed ion-electron conducting metal oxides and hydrous metal oxides, such as RuO2, which store charge via a cation-electron insertion mechanism, as shown in equation (1).
Electrodes based on disordered, hydrous RuO2 yield specific capacitances as high as 720 F/g (Zheng et al., J. Electrochem. Soc., 142, 2699 (1995)). The application of RuO2 is limited however by the high costs of the ruthenium precursors.
Electronically conducting polymers store charge by a doping/de-doping process where electronic state changes in the polymer are compensated by cation or anion incorporation from the supporting electrolyte. Examples of relevant conducting polymers include polyaniline (equation (2)), polypyrrole, polythiophene, polyacetylene, and their derivatives. Because this Faradaic doping process occurs through the bulk volume of the polymer, high energy densities are accessible, as observed with the metal oxides. However, conducting polymers offer the advantage of lower costs relative to those for noble metal oxides. A potential disadvantage for conducting polymers is their somewhat lower conductivities (1-100 S/cm) compared to carbon-based capacitors. The conductivity of such polymers also undergoes modulations as they are electrochemically cycled between the doped (conducting) and de-doped (insulating) state. Conducting polymer electrodes are typically fabricated by electrodepositing thick (up to 10 μm) coatings onto carbon paper, thus their electrical properties may restrict their overall rates of charge and discharge for deep levels of de-doping (A. Rudge et al., J. Power Sources, 47, 89 (1994)).
